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      §4. Pain has the same efficacy and use to set us on work, that
Pleasure has, we being as ready to employ our Faculties to avoid
that, as to pursue this: Only this is worth our consideration, That
Pain is often produced by the same Objects and Ideas, that produce Pleasure
in us. This their near Conjunction, which makes us often feel pain
in the sensations where we expected pleasure, gives us new
occasion of admiring the Wisdom and Goodness of our Maker, who
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designing the preservation of our Being, has annexed Pain to the
application of many things to our Bodies, to warn us of the harm
that they will do; and as advices to withdraw from them. But he,
not designing our preservation barely, but the preservation of every
part and organ in its perfection, hath, in many cases, annexed pain
to those very Ideas, which delight us. Thus Heat, that is very
agreeable to us in one degree, by a little greater increase of it,
proves no ordinary torment: and the most pleasant of all sensible
Objects, Light it self, if there be too much of it, if increased beyond
a due proportion to our Eyes, causes a very painful sensation. Which
is wisely and favourably so ordered by Nature, that when any
Object does, by the vehemency of its operation, disorder the
instruments of Sensation, whose Structures cannot but be very nice
and delicate, we might by the pain, be warned to withdraw, before
the Organ be quite put out of order, and so be unfitted for its proper
Functions for the future. The consideration of those Objects that
produce it, may well perswade us, That this is the end or use of
pain. For though great light be insufferable to our Eyes, yet the
highest degree of darkness does not at all disease them: because
that causing no disorderly motion in it, leaves that curious Organ
unharm’d, in its natural state. But yet excess of Cold, as well as
Heat, pains us: because it is equally destructive to that temper,
which is necessary to the preservation of life, and the exercise of the
several functions of the Body, and which consists in a moderate
degree of warmth; or, if you please, a motion of the insensible parts
of our Bodies, confin’d within certain bounds.
Locke Hum II, 7, §4, pp. 129-130